The hydrotest temperature becomes more problematic as we use more advanced alloys in large boiler.
The newer high temperature creep resistant ferritic alloys ( P91, P92,P112) have very low ductility and low notch toughness at 70 F, and this drops off to near zero toughness below 70F, or if unusual weld electrodes or unacceptable heat treatment was used. Two other items worsen this hydrotest problem.
First, these pipelines have built in stresses , either due to cold springing or lack of effected in-service creep relief of shakedown stresses. So the pipes can be considered to be highly spring loaded, with high stress concentrations at the elbows and at the welds lines to supported valves.
Second, the typical means of filling the unit with hot water from the economizer, thru the drum to the main steam outlet, does not guarantee that the final components( ie, main steam outlet header and Xfer pipe to turbine))will be at a sutiably hot temperature. In particular, if the unit was stored dry and cold in a cold environment, the hydrotest water will probably not be above 70F by the time it reaches and fills the Xfer pipe to teh turbine, and yet it is exactly those final components that have null ductility. To properly hudrotest those unit, one might choose to fill from the outlet to the economizer, and monitor or measuer actual metal temperatures at various locations to ensure no part of the unit is below 70F.